Device for measuring, at a plurality of points, the microwave field diffracted by an object

ABSTRACT

An arrangement for measuring, at a plurality of points, the microwave field diffracted by an object. A microwave radiation source illuminates an object. Electric doublet antennae loaded by diodes are disposed in line along the path of the radiation diffracted by the object. A guide structure, with coupling antennae, is disposed along the line of doublet antennae and collects the diffracted radiation. Electronic circuits control the whole so as to measure the diffracted field at the position of each doublet antenna.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a device for measuring, at a pluralityof points, the microwave field diffracted by an object illuminated by amicrowave radiation source at a frequency and comprising:

at each of said points an electric doublet type antenna loaded at itscenter by a diode,

means for collecting the diffracted microwave radiation and delivering acollected microwave signal,

means for generating a low frequency signal,

multiplexing means, disposed between said generating means and each ofsaid diodes, and

means for controlling said multiplexing means so that at least one ofsaid diodes is biassed by said low frequency signal and, in response tosaid low frequency signal and said collected microwave signal, forgenerating a signal representative of the microwave field at the pointwhere said doublet antenna loaded by said biassed diode is located.

Such a device may be used for numerous industrial applications and inparticular for the non destructive control of objects likely to presentcertain defects, or else materials or products having undergonetransformations such as drying, de-freezing or polymerization, forexample, so as to check the results of such transformation.

2. Description of Related Art

In fact, it is known that a body illuminated by a microwave radiationsource diffracts this radiation in a way which obviously depends on itsshape, but also on the dielectric constant and on the conductivity atevery point of its volume. Since these last two quantities dependobviously on the internal structure of the body, it is thereforepossible to carry out non destructive control of the internal structureof a body by analyzing the diffraction of a microwave field illuminatingthis body.

By diffracted field is here meant either the field transmitted throughthe body or the field reflected thereby, so that the analysis of thediffraction may take place either during transmission, or reflection, orboth at the same time.

Thus, by way of example, the defects of a wooden plank may be analyzed,the degree and homogeneity of drying of a panel or layer of plaster orof a sheet of paper checked, the development and result of thepolymerization of a plastic material checked or the stage of de-freezingof foods evaluated.

A measuring device of this type is already known, described in U.S. Pat.No. 4,552,151. In this device, the doublet antennae are connected end toend so as to form the columns of a "matrix retina", i.e. the assembly ofthe doublet antennae form a rectangular panel. The collector means areformed by a stack of waveguides, each in the form of a horn and theassembly is controlled so that the value of the field may be measuredsuccessively at any point of the panel.

Such a two dimensional device is however fairly cumbersome for thelength of each horn is of the order of size of the largest dimension ofits openings, which substantially defines the maximum analysis length.This device is in addition of a relatively high cost price and it isrelatively complex to use, particularly considering the necessity ofusing a microwave multiplexer. In addition, although it is suitable forbiomedical applications, or laboratory applications, it is ill-fitted toindustrial applications of the above type.

SUMMARY OF THE INVENTION

The object of the present invention is to overcome the above drawbacksby providing a compact measuring device of low cost and with high speedand simple operation, which is well suited to the industrial medium.

For this, the present invention provides a measuring device of the abovetype characterized by the fact that:

said doublet antennae are disposed along at least one line,

said collector means comprise a single microwave guide structure,disposed along said doublet antennae line, on the side opposite saidobject at a distance from said doublet antennae,

along said guide structure is provided a plurality of antennae forcoupling to said guide structure, each coupling antenna transforming themicrowave radiation which it receives into a wave guided by said guidestructure,

one end of said guide structure is provided with a termination and theother end with means for converting said guided waves into saidcollected microwave signal,

said coupling antennae are disposed at positions such that they wouldall radiate in phase if, with said source extinguished, a singlemicrowave signal were applied to said conversion means,

said distance between the doublet antennae and the guide structure isdetermined so that, under these conditions, a uniform field is createdon said doublet antennae line.

For the measuring device of the invention, because the doublet antennaeare distributed over a line, and because the collector means onlycomprise a guide structure disposed along the same line, at a fewwavelengths from the doublet antennae, the size and cost price are low.In addition, in the industrial applications mentioned above, the objectexamined is, in a large majority of cases, an object travelling in frontof the measuring device. The arrangement of the doublet antennae linesubstantially perpendicularly to the travel direction will then make itpossible to obtain in a simple way an image of the object.

With the prior art device, the size in the direction perpendicular tothe analysis plane is of the order of a meter for an analysis length ofthe same order of size, whatever the wavelength, whereas in the deviceof the invention it is of the order of a few wavelengths, namely forexample less than 10 cm for a working frequency of 9 GHz. Furthermore,for the same length of the device along the analysis line, the usefulzone is larger.

In the device of the invention, it is remarkable that, because of theuniformity of the field which would be created by the coupling antennaeon the doublet antenna line, the latter may be disposed at a spacingchosen solely as a function of the fineness of analysis desired in theapplication considered, without having to worry about the periodicity ofthe propagation on the guide structure. In other terms, the spacing ofthe doublet antennae and the spacing of the coupling antennae are chosenindependently of each other, which confers on the measuring device ofthe invention a great flexibility in use.

In addition, the device of the invention makes it possible to measurevariations of the amplitude and phase of the microwave field along thedoublet antennae line, with a view to a direct display or syntheticprocessing or focussing at points inside the object. In the latter twocases, it is thus possible to obtain a tomographic representation of theobject.

Thus, the device of the invention which is very well suited for the nondestructive control of objects or materials travelling past with a viewto controlling their production unit, is also suited for a more thoroughanalysis carried out in the laboratory, for permitting, for example, abetter understanding of the transformation process which the objectundergoes.

Advantageously, the volume situated between said guide structure andsaid doublet antennae is closed by two walls parallel to said line andcovered with a layer of material absorbing the radiation at saidmicrowave frequency.

Advantageously again, between said object and doublet antennae isprovided a matching screen.

The preceding elements make it possible to optimize the device, at themicrowave working frequency, by avoiding leaks and parasite reflectionsof the microwave radiation and permitting the matching and tuning of thedoublet antennae array.

BRIEF DESCRIPTION OF THE DRAWINGS

The present invention will be better understood from the followingdescription of several embodiments of the measuring device of theinvention, with reference to the accompanying drawings in which:

FIG. 1 is a perspective view of the device of the invention used fordetecting defects in wood;

FIG. 2 shows a variant of construction of the guide structure and of theside walls of the measuring device of FIG. 1;

FIG. 3 shows a matching screen for the doublet antennae of the measuringdevice of FIG. 1;

FIG. 4 is a variant of construction of the matching screen of FIG. 3;

FIGS. 5, 6, 7, 8 and 9 each show a variant of the guide structure and ofthe doublet antennae of the measuring device of FIG. 1; and

FIG. 10 shows in greater detail a variant of the arrangement of thedoublet antennae of the device of FIG. 1.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

Referring to FIG. 1, a measuring system for detecting defects in awooden plank 1 will be described. Plank 1 travels, as shown by arrow 11,in front of the fixed measuring system adapted for detecting the defectsin the plank so as to control its subsequent cutting in an optimum way,for example.

A microwave radiation source, in this case a horn 2, is fed with amicrowave signal ME delivered by a microwave generator 21. The microwavesignal ME is a signal at a single frequency.

Horn 2 illuminates plank 1 over the whole of it width. A device formeasuring the microwave field diffracted by the plank 1, whichdiffracted field is here the field transmitted through this plank 1, istherefore disposed opposite horn 2, on the other side of plank 1, at adistance magnified in the figure for obvious reasons of clarity.

This measuring device comprises first of all a plurality of antennae 3,of electric doublet type, i.e. each comprising two equal and alignedrectilinear sections. The cathode of a diode 4 is connected to one ofthe sections whereas its anode is connected to the other section, sothat each of the doublet antennae 3 is loaded at its center by diode 4,here of PIN type. The doublet antennae 3 are disposed along a line, hereperpendicular to the travel direction 11 of the plank, and correspondingto the elongate zone illuminated by the horn 2.

A guide structure, here a rectangular guide 5, is disposed along theline of doublet antennae 3, on the side opposite plank 1 and at adistance D equal, as will be seen further on, to a few wavelengths, atsaid microwave frequency and in the medium in which the operation takesplace, here air.

Slots 55, whose geometrical characteristics will be described in greaterdetail hereafter, are formed in the large side of guide 5 which issituated opposite the line of doublet antennae 3.

At one end of guide 5 is mounted a termination 51, here a short circuit,whereas at the other end is mounted a transition 52, of the knowncoaxial guide type.

Two walls 53, rigid and covered with a layer 531 of material absorbingthe microwave radiation at the working frequency, are disposed parallelto the line of doublet antennae 3, on each side of guide 5 for closingthe volume situated between guide 5 and the doublet antennae 3. Theabsorbing material used is for example a plastic material foam chargedwith carbon of current type, for reducing the multiple reflectionsbetween guide 5 and the doublet antennae 3.

As shown in FIG. 1, each doublet antennae 3 has a section connected tothe electric ground of the system, here for example by welding to one ofthe walls 53, which is for this purpose made from metal and grounded.The other section of each doublet antennae 3 is here connected to eachof the outputs of a multiplexer 7 which receives at its single input alow frequency signal B from a generator 6.

By "low frequency signal" is here meant a periodic square of sinusoidalsignal whose frequency range is considerably lower than the range offrequencies usually called microwave. Thus, since we can say that themicrowave range is limited downwards to a few hundred MHz, the signal Bwill in general have a frequency less than a few tens of MHz.

An electronic circuit 8, controlled by a microprocessor, is providedwith two microwave inputs receiving the microwave signal ME and amicrowave signal MC delivered by the transition 52, a low frequencyinput receiving the signal B, a digital output delivering a signal C forcontrolling the multiplexer and an output delivering a signal SI,representative of the microwave field measured and applied for exampleto a display device, not shown because it is known.

The device which has just been described operates as follows.

At all times, the microprocessor of the electronic circuit 8 controlsthe multiplexer 7 so that signal B is applied to a single one of thediodes 4. Thus, only this diode is biassed, successively reversely andforwardly, at the timing of signal B.

Simultaneously, the microwave radiation illuminates plank 1 and thefield diffracted at the level of each of the doublet antennae 3 is asample of a microwave image of the slice of plank illuminated.

Each slot 55 acts moreover as a coupling antenna to guide 5, fortransforming the radiation which it receives, and in particular theradiation from the array of doublet antennae 3, into a wave guided byguide 5.

The guided waves from the different slots 55 are converted into themicrowave signal MC by the transition 52. The microwave signal MC iscalled collected microwave signal since it results from the collectionof the radiation received by the different slots 55 and in particularthe radiation from the array of doublet antennae 3.

In the electronic circuit 8 the collected microwave signal undergoessynchronous microwave detection by means of the microwave signal ME,then a second low frequency synchronous detection by means of signal B.Now, among all the collected radiation, only the radiation from thedoublet antennae 3, loaded by the diode biassed by signal B, ismodulated by this signal B. Thus, the signal MC, after microwave thenlow frequency detection is only representative of the field at the pointwhere the doublet antenna 3 is located, loaded by diode 4 which isbiassed by means of signal B. The microprocessor of circuit 8 maytherefore control the successive biassing of each of diodes 4 andelaborate, from the signal C which indicates the position of themeasuring point, and from the signal resulting from the double detectionwhich indicates the value of the field at this measuring point, a signalSI for controlling the display device.

This type of operation, which results from the use of the method knownto a man skilled in the art under the name "modulated diffusion method"is described in the already mentioned U.S. Pat. No. 4,552,151.

This type of operation is not the only one which may be considered forthe device of the invention. Thus, by providing on each of the outputsof multiplexer 7, a controllable low frequency phase shifter, bycontrolling the multiplexer so that the diodes are biassedsimultaneously, in accordance with the teaching of French patent 86 05205, the device can be focussed on an inner point of a thick plank, thusmaking possible a tomographic type representation.

The electronic circuit 8, which therefore comprises mainly a controlmicroprocessor and a synchronous microwave detector followed by asynchronous low frequency detector, is within the scope of a man skilledin the art and will not be further described.

FIG. 2 shows a variant of construction of the side walls of the deviceof the invention. In this Figure, guide 5a is narrower than guide 5 andthe side walls 531a widen out and they end in two metal flanges 532extending in the plane of the doublet antennae 3.

In FIGS. 3 and 4 are shown two variants of construction of a matchingscreen which may be disposed between plank 1 and the doublet antennae 3.

In FIG. 3, a metal plate 31a is provided with openings 310 disposedopposite each doublet antennae 3. The size of the openings and theirdistance to the doublet antennae are determined and adjustedexperimentally, taking into account the application considered so as toobtain good matching of the antennae, i.e. to make the microwave powerreflected by the doublet antennae towards the object examined minimum.

In FIG. 4, a single thick dielectric plate 31b is used as matchingscreen. Here again, the dielectric constant of the plate, as well as itsthickness, are determined and adjusted experimentally so as to obtaingood matching.

FIG. 5 shows in greater detail the arrangement of slots 55 in guide 5 ofFIG. 1. Slots 55 are here formed in the large side of guide 5 and thedoublet antennae 3 are all perpendicular to the line along which theyare disposed. Slots 55 are longitudinal, of length l and are disposedalternately on each side of the median axis 70 of the large side of theguide, at a distance s from this axis 70. Two successive slots areseparated by a guided wave half-length, namely λg/2 at the workingfrequency. They have a length close to the radiated wavelength, namelyλ/2, at the working frequency, and small width, but whose value is notcritical.

The operation of the coupling antennae formed by means of slots 55 iseasy to understand and to characterize if we assume that horn 2 is notradiating and if we assume that a microwave signal is applied to thetransition 52. Although the use for which the device of the invention isprovided does not correspond to this operating assumption, we come to itby the theorem of reciprocity, well known to a man skilled in the art.Such an assumption facilitates understanding of the operation of theslots.

If therefore we assume that a microwave signal is applied to transition52, the guided wave in guide 5 is in phase opposition opposite each ofthe successive slots 55, since they are spaced apart by λg/2. However,because of their alternation with respect to the median axis 70, slots55 all radiate in phase. In addition, they are fairly close together soas to create a uniform field along the doublet antennae line 3, disposedat the distance D of a few radiated wavelengths. In this case, and asalready mentioned, the doublet antennae 3 may be spaced apart by adistance which is the best suited to the analysis of the object,independently of the periodicity of propagation in guide 5. Here, anddepending on the desired fineness of analysis, the spacing between thedoublet antennae 3 is between a few millimeters and a few centimeters.

The distance s may be determined for example in the following way. Ifguide 5 includes in all N slots having a reduced conductance equal to g,good matching requires:

    g=1/N

and reference is then made to charts for determining, as a function ofthe reduced conductance g, the value of distance s. Such charts are wellknown to a man skilled in the art and are available in particular in thefollowing work: "Antenna Engineering Handbook", Ed, Henry JASIX, Chap.9; Slot antenna arrays, MacGraw Hill, N.Y., 1984.

As shown in the lower part of FIG. 5, slots 55 are equivalent, in theplane xOy of the large side of guide 5 to magnetic doublets mperpendicular to the electric doublet antennae 3.

FIG. 6 shows a variant of construction in which the doublet antennae 3bare all parallel to the line along which they are disposed. Arectangular guide 5b, in the small side of which are formed slots 55b,is disposed so that these slots 55b are opposite the doublet antennae3b. Slots 55b are spaced apart by λg/2 and form, with the median axis70b of the small side equal and alternately opposite angles. Slots 55bact like coupling antennae equivalent to magnetic doublets m_(b). Theywould radiate in phase if a microwave signal were applied to guide 5band would then create a field whose resultant, in the direction of thedoublet antennae 3b, would be uniform at the level of these doubletantennae 3b.

In FIG. 7 is shown another variant of the device of the invention. Thedoublet antennae 3b remain oriented as in the preceding example, namelyparallel to their alignment but the guide structure is now a microstripline 5c which includes, in a way known per se, a dielectric plate 58,having one face coated with a metallic layer connected to the electricground, the other face supporting a conducting microstrip 56c. As shownin FIG. 7, microstrip 56c is widened periodically so as to formrectangular metal patches 57c connected therefore in series to themicrostrip line 5c. It is known that the edges 55c, perpendicular tomicrostrip 56c, of rectangles 57c are equivalent, because of thedeformation of the field lines, to magnetic dipoles m_(c) which are hereperpendicular to the doublet antennae 3b. Line 5c is provided so thatthe edges 55c are spaced apart by λg/2, so that, as for the slots 55 ofguide 5, they all radiate in phase when a microwave signal is applied tothe microwave line 5c.

FIG. 8 shows another variant in which the guide structure is themicrowave line 5d, but which is adapted so as to be suitable for thedoublet antennae 3, oriented perpendicularly to their alignment. In FIG.8, rectangular metal plates 57d are shunt connected to microstrip 56d ofline 5d, but perpendicularly to this line 5d, so that the two edges 55d,parallel to line 5d, of plates 57d act like magnetic doublets m_(d)perpendicular to the electric doublet antennae 3. Here again, line 5d isdesigned so that the coupling antennae 55d are fed in phase by a signalapplied to line 5d. Here, the shuts are spaced apart by a guidedwavelength λg and the edges 55d are spaced apart by λg/2.

The operation of these patch antennae is described in the work alreadymentioned "Antenna Engineering Handbook", Ed Henry JASIX, Chap. 7;Microstrip antennas, MacGraw Hill, N.Y., 1984.

It should be noted that, although it is practical to place the couplingantennae so that they are spaced apart by a guided wave half length,while adjusting the symmetries of their positions so that they radiatein phase, this is not obligatory and they may be placed for example sothat they are spaced apart by a guided wavelength, by disposing them atidentical positions. The only criterion to be respected is that thecoupling antennae must be disposed so as to all radiate in phase, atpositions which are sufficiently close so that, under these conditions,a uniform field is created on the doublet antennae line 3, the distancebetween these doublet antennae and the guide structure remainingsufficient to avoid coupling phenomena.

In FIG. 9 is shown a variant of the device of the invention, which makespossible the measurement of the diffracted field along two orthogonalpolarizations. The antennae 3e of a first series are oriented at 45°with respect to the median axis 70, loaded by diodes 4e and connected toa multiplexer 7e receiving a low frequency signal Be, of frequency Fe.The antennae 3f of a second series are oriented perpendicularly toantennae 3e, loaded by diodes 4f and connected to a multiplexer 7freceiving a low frequency signal Bf, of frequency Ff. In a way notshown, the collected microwave signal, instead of undergoing a singlesynchronous low frequency detection, undergoes two synchronousdetections, one at frequency Fe and the other at frequency Ff, whichmakes possible the simultaneous determination of the two components ofthe field at one point.

It would also be possible to use only a single multiplexer with twicethe number of channels, by successively biassing each of the diodes 4eand 4f so as to determine successively the two components of the fieldat one point.

In FIG. 10 has been shown in greater detail an arrangement of thedoublet antennae 3 and their connection to multiplexer 7. A singledielectric plate 38 bears on the upper end of walls 53 covered withtheir absorbing layer 531. Two metal layers 36 are provided under plate38, on each side of a zone in which the doublet antennae 3 are situated.On the upper face of plate 38, the two sections 31 and 32 of eachdoublet antenna 3 are formed by means of a conducting microstripsupported by plate 38.

The PIN diode 4 is connected to these two sections 31 and 32 which areeach extended as a low pass filter 371 and 372 respectively, formed in away known per se, above the metal layers 36, by varying the width of themicrostrip so as to obtain alternation of narrow microstrip portions andwide microstrip portions. The low pass filters 371 are each connected toan output of multiplexer 7 and the low pass filters 372 are allconnected to the electric ground. The low pass filters 371 and 372behave like short circuits for the low frequency signal B and like opencircuits for the signals at the microwave frequency.

In the preceding description, the case has always been considered inwhich the object has the microwave radiation passing therethrough, i.e.in the case where the diffracted microwave field is the transmittedfield. This is not obligatory and it is also possible to measure, withthe device of the invention, the field reflected by an objectilluminated by microwave radiation or simultaneously the transmittedfield and the reflected field, using if necessary several devices.

Similarly, in FIG. 1, a horn 2 has been shown for illuminating plank 1.This is not obligatory and it would be possible to use advantageously,instead of horn 2, a microwave radiation source comprising a guidestructure having coupling antennae, identical to the one used for thecollector but obviously without any doublet antenna. Such a guidestructure is then placed at a few wavelengths from the object to beilluminated.

For some applications, it is advantageous to use a microwave radiationsource with double biassing.

It is not obligatory to use PIN diodes and they may be replaced byphotodiodes which can be biased by a laser beam or optical fibers.

Similarly, the guide structure such as guide 5, 5a, 5b and lines 5c and5d are not necessarily terminated by a short circuit. Thus, they areoften terminated by a matched load, as is known.

In the case where the object is not travelling, and when it is desiredto have a two dimensional image of the object, it is obviously possibleto juxtapose several devices in accordance with the above device.

The procedure for experimentally determining the distance D is thefollowing, with reference to FIG. 1. With no object such as plank 1interposed between horn 2 and the doublet antennae 3, these latter areilluminated uniformly. While circuit 8 controls successively thebiassing of each of diodes 4 by means of signal B, as has beendescribed, guide 5 is moved away from a position in which it is situatedagainst the doublet antennae 3, while observing the displayed signal. Atthe beginning, oscillations are to be found corresponding to theperiodicity of distribution of slots 55, but these oscillations areattenuated gradually as distance D increases. At the same time, thelevel of the received signal decreases. It is stopped when the field isuniform on the doublet antennae line, i.e. when the residual undulationsare small enough to be compatible with the required precision. Ingeneral, this result is obtained for a distance D of the order of a fewwavelengths. It will be noted that, by suitable calibration, the defectsof the device can be corrected.

Naturally, the device may be used for testing the field radiated byindustrial applicators and telecommunications or radar antennae,possibly provided with radomes.

What is claimed is:
 1. An arrangement for point-by-point measuring amicrowave radiation field diffracted by an object to be analyzed,comprising:(a) means for irradiating an elongated zone on the objectwith microwave radiation along a predetermined direction to generate themicrowave radiation field to be measured; (b) a plurality of discrete,electric doublet antennae spaced apart of one another in a first planealong said predetermined direction, each doublet antenna having acenter-loaded diode; (c) multiplexing means for successively biasing thediodes of the doublet antennae, each diode in its turn; (d) a single,elongated microwave guide extending between opposite guide ends alongsaid predetermined direction; (e) a plurality of discrete, couplingantennae spaced apart of one another along said predetermined directionin a second plane generally parallel to, and spaced at a distance from,said first plane, to successively couple into the guide microwaveradiations impinging on the coupling antennae during successive biasingof the diodes of the doublet antennae; (f) means for terminating one ofthe guide ends; and (g) means at the other of the guide ends forconverting all the microwave radiations in the guide to a single outputsignal indicative of the microwave radiation field being measured. 2.The arrangement according to claim 1, wherein the irradiating meansincludes a microwave generator and a horn connected to the generator,said horn having a radiating aperture extending along the predetermineddirection.
 3. The arrangement according to claim 2; and furthercomprising a pair of walls extending along said predetermined directionbetween said first and second planes, and a layer of microwave radiationabsorbent material on the walls.
 4. The arrangement according to claim3, wherein the walls are constituted of a metal material, and have metalflanges co-planar with said first plane.
 5. The arrangement according toclaim 1; and further comprising an impedance matching screen betweensaid first plane and the object.
 6. The arrangement according to claim5, wherein the matching screen is a metal plane having openings disposedopposite each of the doublet antennae.
 7. The arrangement according toclaim 5, wherein the matching screen is a thick plate of dielectricmaterial.
 8. The arrangement according to claim 1, wherein the guide isa hollow, metal waveguide having walls, and wherein the couplingantennae are slots formed in one of the walls of the waveguide.
 9. Thearrangement according to claim 8, wherein the waveguide supports thepropagation of an electromagnetic wave at a guide wavelength, andwherein the slots are spaced apart at one-half the guide wavelength. 10.The arrangement according to claim 8, wherein the guide has a medianline extending along the predetermined direction, and wherein the slotsextend parallel to, and laterally offset at a spacing from, the medianline.
 11. The arrangement according to claim 8, wherein the guide has amedian line extending along the predetermined direction, and wherein theslots are located on, and inclined relative to, the median line.
 12. Thearrangement according to claim 1, wherein the guide is a microstrip lineon a substrate, and wherein the coupling antennae are edges ofconductive regions on the substrate.
 13. The arrangement according toclaim 1, wherein each doublet antenna includes two elongated sectionsextending along a common line at opposite ends of the respectivecenter-loaded diode, and wherein the common line is generallyperpendicular to said predetermined direction.
 14. The arrangementaccording to claim 1, wherein each doublet antenna includes twoelongated sections extending along a common line at opposite ends of therespective center-loaded diode, and wherein the common line is generallyparallel to said predetermined direction.
 15. The arrangement accordingto claim 13, wherein the coupling antennae are equivalent to magneticdoublets perpendicular to the common line.
 16. The arrangement accordingto claim 1, wherein the multiplexing means includes a generator forgenerating a timing signal; and a multiplexer having an input forreceiving the timing signal, and a plurality of outputs respectivelyconnected to the doublet antennae.
 17. The arrangement according toclaim 16, wherein each doublet antenna includes two elongated conductivesections at opposite ends of the respective center-loaded diode, andfurther comprising a first plurality of low pass filters respectivelyconnecting the outputs of the multiplexer to one of the sections of arespective doublet antenna, and a second plurality of low pass filtersrespectively connecting the other of the sections of a respectivedoublet antenna to ground.
 18. The arrangement according to claim 17,wherein the sections of each doublet antenna, the low pass filter andthe guide are all constituted of a single microstrip.
 19. Thearrangement according to claim 1, wherein the doublet antennae arearranged in two groups, each doublet antenna of one group extendinggenerally orthogonally to a respective doublet antenna of the othergroup.
 20. The arrangement according to claim 1, wherein the terminatingmeans is a matched load.
 21. The arrangement according to claim 1,wherein the terminating means is a short circuit.